The present invention relates to an illumination optical system for illuminating a target plane using the light emitted from a light source. The present invention also relates to an image display apparatus, such as a liquid crystal projector, which uses the illumination optical system to illuminate an image display element, such as a liquid crystal panel, and to project the light from the image display element onto the target device to be projected, such as a screen.
Various projectors have recently been proposed, each of which is configured to enlarge and project onto a screen etc. through a projection lens, a light that has been modulated by an image display element, such as a liquid crystal light bulb, in accordance with image data. It is important for the projector that the image projected onto the screen has a uniform brightness over the entire screen.
FIG. 9 is a schematic view of principal part of an illumination optical system Ea in a conventional projector. In FIG. 9, a light emitted from a light source 101 is emitted as a substantially parallel light by a parabolic reflector 102. This parallel light is split and focused by a first fly-eye lens 103 (which is a lens array in which micro spherical lenses are two-dimensionally arranged).
The respective split and focused lights (such as three lights 101a, 101b, and 101c) are focused near a second fly-eye lens 104, and form an image of the light source 101 or a secondary light source image. The micro lenses in the fly-eye lenses 103 and 104 have a rectangular shape that is similar to a shape of a liquid crystal panel 107 provided on a target plane.
Plural split lights 101a, 101b, and 101c emitted from the second fly-eye lens 104 are condensed by the condenser lenses 105, and plural split lights illuminate the liquid crystal panel 107 in superposition via a color separation optical system 106, etc.
FIG. 9 shows only principal elements necessary to explain functions of the illumination optical system Ea for simplified description purposes.
In general, an attempt to improve the light available efficiency is likely to increase an angular distribution of the light incident upon the target plane in the illumination optical system. Thus, various problems occur, when the illumination optical system uses an optical element sensitive to an angular characteristic. For example, the color separation optical system etc. can use a color separation film, such as a dichroic mirror, and a dichroic prism, which inclines to an optical axis in the illumination optical system, and a polarization separation film, such as a polarization beam splitter (“PBS”). In that case, the brightness and colors become uneven, the contrast lower, and the image quality deteriorates on the target plane.
In order to maintain the image quality, an illumination optical system including an asymmetrical optical system that makes an angular distribution narrow in a direction sensitive to the angular distribution of the optical element and wide in a direction insensitive to the angular distribution is known in Japanese Patent Applications, Publication Nos. (“JPs”) 06-75200 and 2004-45907.
JP 06-75200 uses an optical integrator for the illumination optical system. The optical integrator uses a one-dimensionally cylindrical array, and uses a Koehler illumination for an illumination in a deflecting direction of an optical element that has a high angular sensitivity, such as a dichroic mirror.
In a section in a direction having a high angular sensitivity of a thin-film element, JP 2004-45907 arranges a stop at a pupil position, and reduces the angular distribution in one sectional direction of the light, improving the contrast of the projected image.
The illumination optical system of JP 06-75200 does not provide a superposed illumination on a Koehler-illuminated section at which the cylindrical lens that does not have a refractive power. Therefore, the light bulb plane (liquid crystal plane's target plane) is likely to cause an uneven light intensity distribution. It is thus necessary to use a relatively flat distribution in an uneven illumination distribution, and thus the light available efficiency is low.
The light from the light source to the condenser lens has a small angular distribution, therefore reducing the image quality deterioration effect on the dichroic mirror between them. However, the condenser lens condenses the light just before the light bulb plane, and thus the image deterioration is inevitable due to an optical element having a high angular sensitivity, such as the light bulb and dichroic mirror, arranged after the condenser lens. Moreover, on a section that does not provide a superposed illumination, due to the fluctuation or deterioration of the light source, such as an arc jump, the light intensity distribution changes on the light bulb and becomes uneven on the projected image, when the light emitting distribution has an uneven brightness.
The image display apparatus of JP 2004-45907 is less subject to the light emitting distribution of the light source, due to the illuminations to the section in superposition in two orthogonal directions. However, the light available efficiency significantly lowers since the stop restricts the light.
A principal point position of part of lenses (an optical system between the lens array and the panel), instead of the stop, differs between two sections to provide different angular distributions on these two sections. However, a method for changing a principal point of a collimeter lens, described in the embodiment, causes a lowered brightness and an uneven light intensity due to the unclear illumination area boundary on the liquid crystal panel plane, or causes uneven contrast or colors, since the telecentric condition destroys (in which the exit pupil is located sufficiently far from panel plane).